Antigen is a substance that stimulates the immune system to produce antibodies against it. Antigens can be proteins, polysaccharides, lipids, or nucleic acids. The ability of a substance to induce an immune response is called immunogenicity, while the ability to bind antibodies is called antigenicity. Factors like molecular size, complexity, shape, stability, and foreignness determine a substance's antigenicity. Antigens contain epitopes that antibodies and T cells recognize. Common antigens include microbial antigens from bacteria, viruses, and other microbes, as well as non-microbial antigens like pollen and snake venom.

1. ANTIGEN

2. • INTRODUCTION…..
• Antigen is a substance which when introduced into the tissues of a
susceptible animal, it stimulates the formation of specific
neutralizing substances or antibody.
• The ability of a material to induce an immune response is referred
to as immunogenicity and such material is called as immunogen.
• The word originated from the notion that they can stimulate
antibody generation.
• The modern definition encompasses all substances that can be
recognized by the adaptive immune system.

3. • Immunogenicity is the ability to induce a humoral and/cell
mediated immune response.
• Antigenicity is the ability of a molecule to be recognized by an
antibody or lymphocyte.
• All molecules possessing the property of immunogenicity also
possess antigenicity but the reverse is not true.
• Molecules vary in their ability to act as antigens and stimulate
immune response.

4. ANTIGENIC DETERMINANT
Epitope,or,Antigenicdeterminants, are the portions of antigen
molecules that physically interact with paratopes (combining
sites) of immune response molecules and therefore actually
"determine" antigen specificity

6. • FACTORS DETERMINING ANTIGENICITY…..
• Molecular size
• Complexity
• Shape
• Stability
• Degradability
• Foreignness
• Genotype of the recipient animal
• Dosage and route of administration
• Adjuvants

8. • MOLECULAR SIZE….
• Foreign proteins are some of the best antigen.
• In general, the larger the molecular weight, the better are their
antigenic properties.
• For example, Hapten
• Penicillin and aspirin are not good antigen since their molecular
weight is less than 1000 dalton.

9. • COMPLEXITY…..
• Good antigens have complex structure.
• Large complex molecules can be readily taken up by
macrophages.
• Complex proteins are good immunogen than the lipids,
carbohydrates and nucleic acids.
• Proteins vary in their antigenic power.
• Bacterial exotoxins and egg albumin are powerful antigens.

10. • Haemoglobin is a weak antigen.
• Protamines, histones and gelatin are almost devoid of
antigenicity.
• More complex carbohydrate is immunogenic, especially if bound
to proteins e.g. cell wall antigens of gram-negative bacteria.

11. • SHAPE…..
• A good antigen should have a fixed shape to trigger higher
antibody response.
• The immune system must recognize its shape.
• The highly flexible molecules that have no fixed shape are poor
antigen.
• The surface area display significant determinant sites for good
immune response.

12. • STABILITY….
• Antigen molecule must be stable and rigid.
For example gelatin, a protein known for its instability is a poor
antigen but they become stable when amino acid residue like
tyrosine or tryptophan are incorporated which cross link the
peptide chain.
• Similarly flagellin is a weak antigen and its stability and antigenicity
are enhanced by polymerization.

13. • DEGRADABILITY….
• All foreign materials are not capable of stimulating immune
response e.g. stainless steel pin, plastic heart valves etc.
• The macromolecule which are degradable in nature can act as
antigen. Plastic bags are inert organic polymers, not degradable
and they are not antigen.
• The antigen molecule should be degraded and processed to form
suitable to trigger immune response.

14. • FOREIGNNESS…..
• Antigens are foreign substances.
• The defense cells of the body, normally do not respond to its own
molecule (self antigen).
• In general, the antigenicity of a substance is related to the degree
of its foreignness.
• Antigen from other individual of the same species is less antigenic
than from other species.

15. • Antigens from related species are less antigenic than those from
unrelated (distinct) species.
• Antibodies are not usually produced against the body’s own
components.
• In exceptional cases autoantibodies are produced against own
tissues (antigen).

16. • GENOTYPE OF THE RECIPIENT ANIMAL….
• The genetic control of the immune response is confined to the
genes within the MHC.
• The MHC gene products function to present processed antigen to
T cell thus playing a central role in determining immunogenicity.

17. • DOSAGE AND ROUTE OF ADMINISTRATION….
• An insufficient dose will not evoke an immune response either
because it fails to activate enough lymphocytes or because it
induces a non-responsive state.
• An excessively high dose also can fail to induce a response
because it causes lymphocytes to enter a non responsive state.
• For inducing strong immune response, repeated administrations
(boosters) are required.
• Antigens are generally administered parenterally i.e. by routes
other than the digestive tract.

18. • ADJUVANT
• Adjuvants [adjuvare (Latin) = to help]. Adjuvants are substances that
when mixed with an antigen and injected, it serve to enhance the
immunogenicity.
• They are often used to boost the immune response when an antigen
has low immunogenicity or when only small amount of antigen is
available.
• Adjuvants augment the immune response by one or more of the
following effects.
– Prolong antigenic persistence
– Enhance co-stimulatory signal
– Induce granuloma formation
– Stimulate lymphocyte proliferation non specifically.

19. • e.g: Alum, Aluminium hydroxide, Bacterial LPS, Saponin, IL-12,
Montanide, Freund’s adjuvant etc.
• Freund’s incomplete adjuvant contains a mineral oil and an
emulsifying agent such as mannide monooleate.
• Freund’s complete adjuvant contains in addition heat killed
Mycobacteria tuberculosis and it is more potent than incomplete
adjuvant.

20. • HAPTEN
• Haptens are usually non-protein substances of low molecular weight
having very little or no antigenic property but acquire antigenicity
when they are coupled to a protein (carrier molecule).
• Haptens are incapable of inducing antibody formation by themselves
but can react specifically with antibodies. They are called partial
antigen.
• The term hapten is derived from Greek haptein mean ‘to fasten’.
• A chemical to be tested as a determinant of specificity was attached
to an aromatic amine such as aniline (aniline, p-amino benzoic acid,
p- amino benzene sulphonic acid, p-amino phenyl arsonic acid), then
diazotized and coupled to a protein.
• The resulting product was called a conjugated antigen or azoprotein.

21. • It is possible to study the immune response of a well-defined
chemical by conjugating to a protein molecule.
• Hapten may be complex or simple molecule.
• Complex hapten is large molecular weight, polyvalent compound.
• When they combine with antibody prepared against the complete
antigenic complex (new antigen) a visible precipitation is formed.
• Simple hapten is a low molecular weight, univalent compound. When
they combine with antibody prepared against complete antigenic
complex, no visible precipitation is formed.
• Examples: Penicillin, Dinitro Phenyl (DNP) etc.

22. • ANTIGENIC DETERMINANTS OR EPITOPES
• An antigen will evoke immune response in a specific host.
• The antigenic specificity is determined by Epitopes, the smallest unit
of antigenicity.
• Epitopes are some specific areas or chemical groupings with steric
(spatial) configuration present on the surface of antigen molecule.
• Epitopes are capable of sensitizing an immunocyte and reacting with
its complementary site on the specific antibody or TCR.

23. • The combining area on the hypervariable region of antibody
molecule that corresponds to epitopes is called paratope or idiotope.
On T cells the recognition site is termed as Agretope.
• Epitopes and paratope determined the specificity of immunological
reaction.
• Epitopes are also called antigenic determinants.
• Most epitopes have 6 to 10 amino acids (T cell epitopes) or 10-15
amino acids (B-cell epitopes)

24. Types of antigen
• TYPES ACCORDING TO ANTIGENIC SPECIFICITY
• Species-specific antigen
• An antigen is specific in relation to a specific species of host only.
• Organ specific antigen
• The antigen is specific in relation to a particular organ in a single
species. For example, kidney protein is different from lung protein.

25. • Heterophile antigens
• They are antigens of identical nature present in the cells of some
bacterial species and also in the tissues of different animals.
Antibody formed against one antigen cross-react with other
antigens.
• There is a phenomenon of antigenic sharing.
• Examples:
• Frossman antigens are a group of related heterophile antigens. They
are present in the cell wall of many bacterial species (glycoprotein
with carbohydrate side chain) belong to the genera Streptococcus,
Shigella, Salmonella and Clostridium and also on the surface of red
blood cells (glycoprotein) of horses, sheep, cats and mice.

26. • The animals having Frossman antigens in their tissues do not
develop antibodies because the antigens are considered as ‘self’.
• A heterophile antigen is found in some proteus strains (OX-19, OX
-2, OX-K) and also certain Rickettsial organisms (e.g. Rickettsia
rickettsii). This forms the basis of Weil-Felix reaction. An
agglutination test is used to detect antibody in human beings
against Rickettisal organisms using Proteus antigen.
• Alloantigens (isoantigens)
• These are antigens present in one individual and are antigenic to
some individuals of the same species. The corresponding antibodies
are called alloantibodies.
• Example: Blood group antigen, Transplantation antigen etc.

27. • Syngenic antigen
• The antigens present in the individuals of the same genetic make up.
Example- antigens present in monozygotic twins.
• Autoantigen
• In certain circumstances, own body tissues develop antigenic properties and
antibody formed against the antigen.
• The auto antigens are sequestrated without contact with the lympho-
reticular system but when antigens are released, they provide an
opportunity to produce autoantibody.
• Example: sperm, lens protein etc.
• Super antigen
• A group of molecules that do not have to be processed by antigen
presenting cells in order to activate T cells. Examples – Bacterial antigens
(Streptococcal A antigen).

28. • ANTIGENIC TYPES BASED ON T CELL RESPONSE
• T dependent antigen
• Complex antigens such as RBC, serum proteins etc. They are rapidly
metabolized, require T cell help and produce memory and class
switching.
• T independent antigen
• LPS of E. coli, pneumococcal polysaccharide, flagellin , Lectin, Con A
PHA, Pokeweed mitogen etc., composed of repeating units. They are
slowly metabolized, no memory or class switching.

29. • MICROBIAL AND NON MICROBIAL ANTIGENS
• Microbial antigens
• It includes bacterial antigens, viral antigens and other microbial
antigens.
• Non-microbial antigens
• They are certain dust particles, pollen grain, snake venom, etc.
• BACTERIAL ANTIGENS
• There are two main groups
– Soluble antigens: Some soluble substances produced by the
bacteria, which are excreted into the environment. For
example, toxins, enzymes etc.
– Cellular antigens: They are the structural units of bacterial
cell. Common bacterial antigens are:

30. • Somatic (O) antigen: In gram negative bacteria (Salmonella, E. coli,
Brucella etc.), somatic antigens are composed of lipopolysaccharide
(LPS)–protein complex, which are good antigen and produce good
immune response.
• But O antigens are highly variable (LPS consists of highly variable
Oligosaccharides) and thus immunity against one ‘O” antigen will not
confer immunity against bacteria bearing other ‘O” antigens.
• Capsular (K) antigen: A variety of bacterial species have capsule
(e.g. Bacillus anthracis, E. coli, Salmonella spp. etc) which is antigenic.
Capsule commonly consists of polysaccharides (e.g. K antigen
of E.coli) but some are composed of polypeptides (e.g. Poly-D-
glutamic acid in case of B.anthracis).

31. • Flagellar (H) antigen: Motile bacteria have flagella
(e.g. Salmonella spp., E.coli, Proteus spp.)
• These Flagella is composed of protein (flagellin), which is antigenic.
• Fimbrial (F) antigen: Fimbrial or Pili antigen are present on the surface
of bacteria .
• Spore antigen: Bacterial spores (e.g. Bacillus spp., Clostridium spp.
etc.,) especially the exosporidium is antigenic.

32. • VIRAL ANTIGENS
• Structural components (VP-Viral Protein) of the virus vary in their size
and complexity.
• Capsid protein and envelope (consists of lipoprotein and
glycoprotein) are antigenic.
• Examples, HN protein (glycoprotein) of Newcastle disease virus.

33. • NON-MICROBIAL ANTIGEN
• Some food may contain substances that evoke immune response
and cause allergic reaction.
• Pollen grains, some dust particles when inhaled, cause allergic
reaction.
• CELL SURFACE ANTIGEN
• Mammalian cell surface contain protein molecules within lipid
bilayer.
• These proteins may act as antigen when injected into another
animal of same species or different species.

34. • Important cell surface antigens are
– Blood group antigen
– Major histocompatibility complex (MHC) molecules
– Cluster of differentiation (CD).

35. • BLOOD GROUP ANTIGEN
• The antigens found on the surface of red blood cells are called blood
group antigens or erythrocytes antigens.
• Earlier attempt of blood transfusion often resulted in disastrous
consequences.
• Blood transfusion became scientifically feasible after the discovery
of blood groups by Landsteiner (1900).
• Most of the blood group antigens are either glycoprotein or
glycolipids and they are the integral components of cell membrane.

36. • There are several human blood group system e.g., ABO, MN, Rh,
Lewis, Kell, Duffy, Kidd, Colton etc.,
• Function:
– The ABO antigens in human are anion and glucose transporter
proteins.
– The M and C antigen of sheep RBC are associated with
membrane potassium pump and amino acid transport.

37. • MAJOR HISTOCOMPATIBILITY COMPLEX MOLECULES
• MHC molecules are present on the surface of nucleated cells and
also on red blood cells in some species (class I MHC).
• Some MHC antigens are present on the surface of macrophages,
dendritic cells and B cells (MHC- class II molecules).
• Some MHC are not bound to cell surface but present in the
secretion (MHC- class III molecules)
• They are protein or glycoprotein substances and provoke immune
response .

38. • CLUSTER DIFFERENTIATION
• It refers to different molecules present on the surface of
lymphocytes, which perform specific function, and the receptors
can be identified by monoclonal antibodies.
• Example CD 8+ refers to T Cytotoxic cells.

39. • ABO BLOOD GROUP SYSTEM
• The antigen found on the surface red blood cells (RBC) are called
blood group antigen.
• The ABO system contains four blood groups and it is determined
based on the presence or absence of two distinct antigens.
• RBC of group A carry antigen A, group B carry antigen B, O group have
neither A nor B antigen and group AB carry both A and B antigen.
• Group A is subdivided into A1 (about 80%) and A2 (about 20%). The
four antigens can be distinguished by two
distinct isoantibodies present in the serum.

40. • ‘A’ groups people have anti B antibody, B groups have anti A antibody,
O groups both anti A and anti B antibody, where as AB groups will
neither have anti A nor anti B antibodies.
• When erythrocytes are mixed with serum containing corresponding
antibodies, agglutination occurs (table).
• Isoantibodies appear in the serum by about six months of age and
persist thereafter.

41. • Since O groups do not have either A or B antigen, RBC from this
group of people are not agglutinated by serum of any other blood
groups, thus O groups are used as universal blood donor.
• But serum from O group contain both anti A and B antibodies and
agglutinate erythrocyte of all other blood groups that means they can
receive blood from group O only.
• AB groups do not have anti A or anti B antibodies and used as
universal acceptor.

42. AGGLUTINATION REACTION WITH RBC AND CORRESPONDING
ANTISERUM

43. • Rh SYSTEM AND ERYTHROBLASTOSIS FETALIS
• Rh system
• Levine and Stetson (1939) demonstrated a new antibody in the
serum of women who developed reaction following blood
transfusion and she recently delivered stillborn baby with
hemolytic disease.

44. • Landsteiner and Wiener (1940) found that rabbit antiserum to rhesus
monkey RBC agglutinated RBC of 80% normal people. This antigen or
RBC is called Rhesus or Rh factor.
• Very large number of antigens are detected in Rh system, they cross
react and weakly immunogenic except ‘D’ or Rho antigen which is
most powerful antigen and responsible for majority of Rh
incompatibility reaction.
• Thus people are divided into Rh positive and Rh-negative groups.
• Rh antigens are determined by three pair of closely linked
allelomorphic genes (Cc, Dd, and Ee).

45. • Erythroblastosis fetalis
• When Rh-negative women carries Rh-positive fetus (when
husband is Rh positive), she develops antibodies to Rh antigen as
fetal blood enter to maternal circulation (minor transplacental
leakage occur during pregnancy but during delivery large amount
of fetal blood enter into maternal circulation).
• Mother develops immunity during first pregnancy.
• During subsequent pregnancy, Rh antibodies (IgG class) pass from
mother to fetus and cause haemolysis or jaundice.